1. Field of the Invention
A method is provided for noninvasively evaluating vasoconstriction and vasodilation in vivo.
2. Description of the Related Art
The vasculature of heart, brain and kidney are capable of maintaining constant blood flow over a wide range of perfusion pressures, a phenomenon known as autoregulation. Several factors affect blood flow in the coronary and other vascular beds including arterial pressure, tissue pressure, neural and humoral influences, tissue metabolites, vascular myogenic tone, and the endothelium (reviewed by Konidala and Gutterman, 2004, Progress in Cardiovascular Diseases 46: 349-373). Appropriate vasomotor responses to constrictive and dilatory stimuli in blood vessels are key elements for regulating blood flow. Disturbances in the balance between constrictive and dilatory stimuli can affect organ blood flow (e.g. ischemia) and total peripheral resistance resulting in blood pressure changes (e.g. hypertension).
Under normal physiological conditions, resistance to blood flow may be overcome by the capability of resistance vessels to dilate in response to a number of factors. However, some pathophysiologic conditions including atherosclerosis, thrombosis and endothelial injury affect the capability of the resistance vessels to dilate in response to normal vasodilatory signals. In addition, diseases such as diabetes mellitus, obesity, congestive heart failure and hypertension, certain autoimmune disorders and certain endocrine disorders have adverse effects on dilation of resistance vessels.
Other factors such as endogenous factors, drugs, and diets may interfere with normal arterial vasodilation. For example, methionine and its metabolic byproduct homocysteine and dyslipidemia impair dilatory processes in a number of vessel types (Dayal et al., 2005, Circulation 112: 737-744; Distrutti et al., 2008, Hepatology 47: 659-667; Hansrani and Stansby, 2008, Journal of Surgical Research 145: 13-18; Moat et al., 2006, Journal of Clinical Investigation 36: 850-859). High fat or salt intake produces abnormal vasodilation (Hermann et al., 2003, Circulation 108: 2308-2311; Mahmud et al., 2008, Journal of Pediatrics 152: 557-562). Solubilized cigarette smoke particles as well as a nicotine metabolite have also been shown to block vascular relaxation (Zhang et al., 2006, BMC Cardiovascular Disorders 6:3; Conklin et al., 2001, Journal of Surgical Research, 95: 23-31). Excessive circulating norepinephrine levels, often seen in pheochromocytoma, lead to abnormalities in vasodilation (Hagashi et al., 2002, Hypertension 39: 513-518). Glucocorticoids, cocaine, certain antineoplastics, cyclosporine A, halothane, for example, are known to have adverse effect on normal arterial vasodilation (Tonga et al., 2001, Toxicology Letters 123: 43-50; El-Mas et al., 2007, Biochemical Pharmacology 73: 359-367; Chow et al., 2006, Journal of Clinical Oncology 24: 925-928; Mondo et al., 2006, Clinical and Experimental Pharmacology and Physiology 33: 1029-1034).
Studies have shown that blood vessels exhibit dilatory responses to cholinergic agonists or cholinomimetics when the endothelium is functionally intact (Sugama et al., 2002, Japan Heart J 43: 545-558; Laher et al., 1995, Canadian J Physiology Pharmacology 73: 1669-1673). The vasodilation is due to the stimulation of muscarinic M3 receptors by acetylcholine and other cholinomimetics on the endothelial cells and the subsequent release of the vasodilator nitric oxide (NO), which is the predominant determinant of resting vascular tone (Brown and Taylor, 2001, Pharmacological Basis of Therapeutics, Hardman, J. G and Limbird, 10th edition, pp 155-173; Moody et al., 2001, Pharmacological Basis of Therapeutics, Hardman, J. G and Limbird, 10th edition, pp 385-397). When the endothelial cells are injured or damaged, muscarinic receptor-stimulated NO production is reduced. The reduced amount of NO combined with the stimulation of muscarinic receptors in the smooth muscle cells in the presence of unopposed sympathetic adrenergic tone can not only result in loss of vasodilating response but potentially produce a vasoconstriction in blood vessels. Therefore, the vasodilation stimulated by cholinergic agonists is dependent upon a functionally intact endothelium.
Due to the vascular morphologic similarities between humans and rabbits, rabbits have been used as a study model to measure vasomotor responses to drugs. Laher et al. used isolated blood vessels from kidneys or ears of rabbits to demonstrate that α-toxin selectively impairs the endothelium-mediated vasodilation (Laher et al., 1995, Canadian J Physiology Pharmacology 73: 1669-1673). Recently, Drolet et al. (Drolet et al., 2004, Cardiovascular Ultrasound 2:10) uses ultrasound recording of the abdominal aorta to show that the diameter of the abdominal aorta changed from 1 to 4% in response to acetylcholine. These methods either do not provide an appropriate physiologic environment normally present in vivo or lack sufficient dynamic range or sensitivity to examine efficiently the vasomotor response in vivo. Therefore, there is still a need to develop an efficient and sensitive method for measuring vasomotor response in vivo.
The present application provides a method for evaluating the effects of test compounds on vasomotor responses. In addition, a method is provided for evaluating endothelial effects on vasomotor function which is comprised of a systemic exposure to constrictive agents such as norepinephrine, dilatory agents such as acetylcholine, or other agents that augment or interfere with endothelial signaling, and measuring the lumen of the central ear artery of a rabbit. The animal model of arterial vasomotor function of the present application is useful for evaluating the effects of test compounds on basal vascular tone and endothelium-dependent vasomotor function in vivo.